1. Field of the Invention
The present invention relates to a shock sensor which is suited for an automotive air bag system for protecting an automotive passenger against shocks at a time of collision.
2. Related Art
In recent years, there is a tendency of increasing the number of automobiles which are equipped with air bag systems for protecting drivers or the like against the shocks.
This air bag system is adapted to inflate the air bag at an instant when it detects a collision. This operation is highly dependent upon the performance of a sensor for detecting the shocks of the collision.
As an acceleration sensor for detecting a large change in the velocity to be caused upon a collision of the vehicle, there has been disclosed in U.S. Pat. No. 4,827,091 a system comprising: a cylinder made of an electrically conductive material; a magnetized inertia member so fitted in the cylinder as to move in the longitudinal directions; an electric conductor disposed at the face of at least one end of the magnetized inertia member in at least the longitudinal direction of the cylinder; a pair of electrodes arranged at one longitudinal end of the cylinder and rendered conductive through said conductor when the magnetized inertia member comes into contact with the conductor; and an attraction member arranged at the longitudinal other end of the cylinder and made of such a magnetic material as to magnetically attracting said magnetized inertia member.
In this acceleration sensor, the magnetized inertia is attracted by the attraction member so that it is left stationary at the other end in the cylinder while no or little acceleration is applied to the acceleration sensor.
If a considerable acceleration is applied to this acceleration sensor, the magnetized inertia member is moved against the attraction of the attraction member. While the magnetized inertia member is being moved, an induction current flows through the cylinder to establish a magnetic force for urging the magnetized inertia member in the direction opposite to the moving direction so that the magnetized inertia member is braked to have its moving velocity reduced.
If the acceleration is lower than the predetermined (threshold) value, the magnetized inertia member fails to reach the leading end of the cylinder and stops in the course of its movement until it is returned to the other end by the attracting force of the attraction member.
If the acceleration exceeds the predetermined (threshold) value (e.g., if the vehicle carrying the acceleration sensor collides), the magnetized inertia member reaches the one end of the cylinder. Then, the conducting layer of the leading end of the magnetized inertia member comes into contact with both of the paired electrodes to establish the conduction therebetween. If a voltage is applied in advance between the electrodes, an electric current flows between the electrodes when the electrodes are short-circuited. It is detected in terms of this current that the vehicle has collided.
In the sensor of U.S. Pat. No. 4,827,091, the paired electrodes are connected when the magnetized inertia member moves to its forward limit. In order for the sensor to operate normally for a long time of period, it is necessary that the inner face of the cylinder to slide on the magnetized inertia member should be smooth and free from any corrosion. Moreover, the electrodes and the magnetized inertia member should also be free from any corrosion or rust. Thus, the cylinder has to be made of a highly corrosion-resistive material and to have its inner face elaborately polished or finished to raise the production cost. Moreover, the electrodes and so on have to be plated with gold to prevent the corrosion.